1. Field of the Invention
The present invention relates to a CMOS bandgap voltage reference and, more particularly, to a CMOS bandgap voltage reference which utilizes cascoded MOS devices to provide increased temperature stability of the bandgap voltage reference.
2. Description of the Prior Art
The bandgap voltage reference, since introduced by Widlar, has become widely used as a means for providing a reference voltage in bipolar integrated circuits. In general, the bandgap reference relies on the principle that the base to emitter voltage, V.sub.BE, of a bipolar transistor will exhibit a negative temperature coefficient, while the difference of base to emitter voltages, .DELTA.V.sub.BE, of two bipolar transistors will exhibit a positive temperature coefficient. Therefore, a circuit capable of summing these two voltages will provide a relatively temperature independent voltage reference. One such circuit arrangement is disclosed in U.S. Pat. No. 4,429,122 issued to R. J. Widlar on Feb. 3, 1981. In CMOS technology, the basic Widlar arrangement may be directly applied, since bipolar devices may be created using standard CMOS processes. However, the bipolar devices available in CMOS are not as stable as those directly developed in bipolar technology, and additional control requirements are needed to provide a relatively temperature stable bandgap reference. U.S. Pat. No. 4,287,439 issued to H. Leuschner on Sept. 1, 1981, discloses one exemplary CMOS bandgap arrangement. Here, the circuit utilizes two substrate bipolar transistors with the emitter of one being larger than the other. The transistors are connected in an emitter follower arrangement with resistors in their respective emitter circuits from which a voltage is obtained to generator the bandgap reference. A later arrangement, disclosed in U.S. Pat. No. 4,380,706 issued to R. S. Wrathall on Apr. 19, 1983, relates to an improvement of on the Leuschner circuit wherein an additional transistor is inserted between the output of the amplifying stage and the substrate bipolar transistors to provide an output voltage of twice the bandgap voltage.
There exist many factors which affect the performance of these and other CMOS bandgap references. One factor not addressed by these prior art arrangements is the temperature dependence of the resistors used in association with the substrate bipolar transistors to provide the needed ratio between the emitter currents. Therefore, true temperature stability cannot be achieved without addressing this problem. One solution is disclosed in U.S. Pat. No. 4,375,595 issued R. W. Ulmer et al on Mar. 1, 1983. In the Ulmer et al arrangement, switch capacitors are used at the inputs associated with V.sub.BE and .DELTA..sub.VBE to sample both voltages. Proper selection of the capacitor ratio provides a weighted sum of both voltages to the amplifier inputs which will be substantially independent of temperature. This particular solution to the resistance-related temperature coefficient problem, however, requires an external clock source and relies on the proper selection of the capacitor values used. The need remains, therefore, for a CMOS bandgap reference which provides increased temperature stability in relation to the resistor-based temperature coefficient which is relatively easy to implement and does not require external circuitry.